There has been conventionally known a mixing device that has a moving bed forming unit for forming a cylindrical moving bed by continuously dropping a particulate catalyst from the top to the bottom while distributing the particulate catalyst in an annular form, an internal stock oil supply part for supplying stock oil over the entire area in a circumferential direction of an inner circumferential surface of the moving bed formed by the moving bed forming unit, and an external stock oil supply part for supplying the stock oil over the entire area in a circumferential direction of an outer circumferential surface of the moving bed formed by the moving bed forming unit (see, for example, Patent Literature 1). In this mixing device, because the moving bed forming unit forms a catalyst into the cylindrical moving bed, the contact area between the stock oil and the catalyst increases, whereby the stock oil and the catalyst can be mixed promptly and evenly.    Patent Literature 1: Japanese Patent Application Publication 10-249178